This invention relates to a method for the automatic setting of the optimum operating point of a d-c voltage source having an internal resistance, as well as apparatus for automatically setting, such a point.
Such a d-c voltage source may be, for instance, a storage battery, a thermocouple, a fuel cell or, in particular, a solar generator. Apart from the fact that the power delivered by these d-c voltage sources depends on non-electric parameters such as the ambient temperature, the internal temperature, the state of charge in the case of storage batteries or the incident power in the case of solar generators, it is a common feature of these d-c voltage sources that there is a definite physical relationship between their two electrical state variables (output voltage and output current) which is usually described by an internal resistance in an equivalent circuit diagram. If, therefore, electric power is taken from these d-c power sources by a d-c control element (chopper), a voltage transformer or another matching transformer, and is fed to a load connected thereto, the more current which is taken off via the voltage transformer, the more the theoretically obtainable miximum output voltage drops. If on the other hand, the voltage transformer is controlled or regulated in such a manner that a given output voltage of the d-c voltage source is maintained, the current which can be taken off is determined thereby. The voltage source has only one electrical degree of freedom, which can be preset as the operating point of the voltage source or the matching transformer. The power output of such a voltage source is a function of the corresponding degree of freedom, i.e., of the operating point, which has its maximum generally at a certain value which represents the optimum operating point ("Maximum Power Point", MPP) with respect to the utilization of the voltage source. Especially in voltage sources, the primary energy of which is free (for instance, solar energy) or practically free as compared to the cost of installation, it is desirable, for optimum utilization of the system, to have the system always run practically at full load, i.e., to always operate at the MPP, in order to supply as much electric energy as possible from the d-c voltage source into a load, for instance, an energy accumulator, such as a battery.
Among the loads which must be considered, in connection with a d-c chopper or another d-c voltage transformer, are consumers such as the on-board network of a vehicle. A d-c voltage transformer can also be used in cases such as a charging controller for a storage battery, with the storage battery followed by a controlled inverter which supplies, for instance, the bus bar of an "insular network", i.e., of a remote group of consumers which is not supplied from the public supply network. If instead of a d-c voltage transformer a controlled inverter (generally, a controlled power converter) is used to convert the primary energy taken from the d-c voltage source into another form of electric energy in a controlled manner, then a-c consumers such as feed pumps which are used for further energy conversion, for instance, the conveying of heat energy of a medium, must also be considered as consumers.
It is already known from DE-OS 29 03 559 to control the power consumption of a load by means of a d-c transformer connected to a solar generator, where the d-c voltage transformer is fed a control voltage by means of which the output voltage of the solar generator is to be controlled to the voltage value corresponding to the optimum voltage value. Accordingly, the control voltage is formed by the control deviation of the generator output voltage from a reference voltage, where the reference voltage is furnished by a solar cell of similar construction, but which is open-circuited, in order to take influences of non-electrical environmental variables into consideration. In other words, for an electrical state variable of the d-c voltage source determining the operating point of a solar generator, a reference value is pre-set, by means of which the power input of a controllable power transmitter connected to the d-c voltage source is controlled or regulated. However, the influence of a change in the operating point due to the current flowing from the d-c voltage source (also designated as a "panel") with its declining characteristic cannot be taken into account sufficiently by the artificial reference voltage formed by the unloaded measuring cell. In addition, spread from unit to unit due to manufacturing tolerances leads to incorrect adjustments of the operating point.
In the known device, the optimum operating point can no longer be found at all if the solar generator or the unloaded measuring cell supplying the reference voltage is partially in the shade or is dirty.